As it is well known in this specific technical field, in many audio applications, which require analog audio signals to be amplified, operational amplifiers inserted as an input stage of an amplifier in the AB class are widely used.
For a more detailed description of this kind of amplifier, reference is made, for example, to the U.S. Pat. No. 6,804,477.
In view of increasingly reducing the electric power consumption of any electronic device, particularly for applications on portable devices, operational amplifiers should be biased with lower and lower current levels.
In this context it must be considered that, by operating in the A class, the signal amplitude should always be lower than the bias current/voltage levels.
However, this need involves a limitation of the overall amplifier performance.
Very briefly, the biasing in the AB class allows the bias current/voltage levels of the different stages composing the amplifier to be dynamically changed, and according to the amplitude of the applied signal, considerably improving the overall efficiency and performances.
More particularly, input amplifier stages in the AB class allow the slew-rate parameters of the whole amplifier to be increased.
The here-attached FIG. 1 shows a traditional example, taken from the known prior art, of an input amplifier stage of the AB class.
In this example, it can be noted how a differential cell 1 receiving differential input signals I, I, could be coupled to other amplifier circuit portions by means of a series of current mirrors being conveniently located along the input-output signal path.
Therefore, a positive bias current can always be added to the differential stage and proportionally to the absolute value of the applied signal.
On the contrary, this bias current falls to the lowest values when no signal is applied.
Although advantageous in many aspects, this known technical solution does not allow the highest bias current to be precisely determined for applications in AB class.
Moreover, the known structure is quite complex and it requires a plurality of current mirrors to obtain an additional current always having a positive value.